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CLI (Command Line Interface): It is basically a command-line program that usually ACCEPTS text as input to execute or run functions of the operating system. It allows users to type declarative commands simply to GIVE instructions to the computer to perform or execute operations. It usually requires LESS memory to use as compared to other interfaces as WELL as it does not require Windows and a low-resolution monitor can be used. It usually provides greater flexibility of use and can also be used to perform things easily that are most difficult to do with GUI.  

GUI (Graphical User Interface): It is basically a human-computer interface that allows users to interact with electronic devices through graphical icons and visual indicators. The use of these graphical elements or icons makes it easier for users to interact with the system. It is visually intuitive and allows higher productivity. It is usually a COMBINATION of graphical and textual interaction that uses menus, buttons, message boxes, etc. 

The first PROCESS STARTED by the KERNEL in LINUX is “init” and its process ID is 1.

INODE: It is a unique name given to each file by OS. Each inode has a unique inode number within a file SYSTEM. It stores various information about files in LINUX such as ownership, file size, file type, access mode, number of links, etc.  

Process Id (Identifier): It is a unique Id given to each process. It is simply USED to uniquely IDENTIFY an active process throughout the system until the process terminates.

Shell SCRIPT, as NAME SUGGESTS, is a script especially written for shell. Here, script means PROGRAMMING language that is being used to control applications. It simply allows the execution of different commands that are entered in the shell.  It generally helps you to create complex programs containing conditional statements, loops, and functions. It is very easy to debug, can simplify everyday automation PROCESSES, and is much quicker as compared to writing big programs.

Load average, as the name suggests, is the average SYSTEM load on Linux SERVERS being calculated over a given period of time. The load average of Linux servers can be found using “top” and “uptime” commands. It is simply used to keep track of system resources. It is REPRESENTED by a DECIMAL number starting at 0.00.  It tells you the load that the system has been under.

Cron: It is a program in Linux that is used to execute TASKS at a scheduled time. It works EFFECTIVELY on machines that run continuously. 

Anacron: It is a program in Linux that is used to execute tasks at certain intervals. It works effectively on machines that are powered off in a day or week. 

Zombie Process, also referred to as a defunct or DEAD process in Linux, is a process that has finished the EXECUTION, but its entry remains in the process table. It usually happens DUE to a lack of correspondence between parent and child processes. This process occurs for the child process because the parent process needs to READ the status of the child process. Once it is completed using the WAIT system call, this process is removed from the process table. 

The daemon that CONTROLS that PRINT spooling PROCESS is the Line PRINTING daemon. 

DAEMONS also referred to as the background process, is a long-running Linux PROGRAM that runs in the background. They do not have any controlling terminal, therefore, they run in the background. These are the processes that are generally started when the system is BOOTSTRAPPED and TERMINATE or end only when the system is shut down. It is simply the way of extending the functionality of the base OS. It provides and offers several functions that are not available in OS. Its main purpose is to handle periodic requests and then forward the requests to the APPROPRIATE programs for execution.  

Proc file system is a pseudo or VIRTUAL file system that provides an interface to the KERNEL DATA STRUCTURE. It generally includes useful information about processes that are running currently. It can also be used to CHANGE some kernel parameters at runtime or during execution. It is also regarded as a control and information center for the kernel. All files under this directory are named virtual files. 

LVM (Logical Volume Management) is basically a tool that provides logical volume management for the Linux kernel. It is being introduced simply to make physical storage device management easier. It also includes allocating disks, striping, mirroring, resizing logical volumes. Its main advantages are increased abstraction, FLEXIBILITY, and CONTROL. It simply allows for flexible disk space management. It is especially required to resize the size of the file system ONLINE. In Linux, the size of the LVM PARTITION can be extended USING “lvextend” command and can be reduced using “lvreduce” commands, respectively.  

FILE that is USED to AUTOMATICALLY mount file systems is a Fstab file. 

There are THREE owners in the Linux System i.e., user, group, and others. These owners have three types of permissions defined as LISTED below: 

• Read (r): It ALLOWS the user to open and read the file or LIST the directory. 
• Write (w): It allows the user to open and modify the file. ONE can also add new files to the directory. 
• Execute (x): It allows the user to execute or run the file.  One can also lookup a specific file within a directory. 

The typical size for a swap partition under a LINUX system should be TWICE the amount of physical MEMORY or RAM available on the system. 

Linux that is SPECIALLY DESIGNED by SUN MICRO system is Solaris.

The MAXIMUM length for a filename under LINUX is 255 BYTES. 

VI editor (VISUAL Editor) is basically a default text editor that usually comes with most of the Linux OS. There are basically three types of MODES used in VI editor as given below: 

• Command Mode/Regular Mode: It is the default mode for the vi editors. It is GENERALLY used to TYPE commands that usually perform particular or specific vi functions. To enter this mode from another mode (Insert mode), one must PRESS [esc]. In simple words, it lets you view the content.  
• Insertion Mode/Edit Mode: This mode allows you to do text editing, or type text into a file. To enter this mode from another mode (command mode), one must press [esc]. In simple words, it lets you delete or insert text or content. 
• Ex Mode/Replacement Mode: This mode is generally used to save the files and execution of the commands. It basically executes files with different parameters. To enter this mode, one must press [:]. In simple words, it lets you overwrite content or text. 

Linux shell is a user interface present between user and kernel. It is used for executing commands and communication with Linux OS. Linux shell is basically a PROGRAM used by users for executing commands. It accepts human-readable commands as input and CONVERTS them into kernel understandable language. 

Different types of shells are commonly used on TYPICAL Linux system as listed below: 

• CSH (C Shell)
• KSH (Korn Shell)
• BASH (Bourne Again Shell)
• TCSH
• ZSH
• Bourne Shell

Linux Process is a type of process that can be in a NUMBER of different STATES. The process enters these states from start till end. Process states in Linux are as follows:  

• New/Ready: In this state, a new process is created and is ready to run.
• Running: In this state, the process is being executed.
• Blocked/Wait: In this state, the process is waiting for input from the user and if doesn't have resources to run such as memory, FILE locks, input, then it can remain in a waiting or blocked state.
• Terminated/Completed: In this state, the process has completed the EXECUTION or terminated by the OS.
• Zombie: In this state, the process is terminated but information regarding the process still EXISTS and is available in the process table.

SWAP space, as the name suggests, is basically a space on a hard DISK that is used when the amount of physical memory or RAM is full. It is considered a substitute for physical memory. Its main function is to substitute disk space for RAM memory when real RAM does not have enough space to HOLD all programs that are EXECUTING, and more space is required. In simple words, it can be used as an extension of RAM by Linux. 

LILO (Linux Loader) is basically a bootloader for Linux that is used to load Linux into MEMORY and start the OS. It is also known as a boot manager that FACILITATES a dual boot of a COMPUTER. It can function as either a master boot program or secondary boot program and PERFORMS various functions such as locating kernel, identifying other supporting programs, LOADING memory, and starting the kernel. If you want to use Linux OS, then you need to install a special bootloader i.e., LILO for it as it allows fast boot of Linux OS. 

There are two types of Linux USER MODE as GIVEN below:  

• Command Line 
• GUI  

A kernel is considered the main component of LINUX OS. It is simply a resource manager that acts as a BRIDGE between hardware and software. Its main role is to manage hardware resources for users and is generally used to provide an interface for user-level interaction. A kernel is the first PROGRAM that is loaded whenever a COMPUTER system starts. It is ALSO referred to as low-level system software.

Its other main functions include: 

• Memory Management
• Process Management
• Device Management
• Storage Management
• Manage access, and use of various peripherals that are connected to the computer.

BASH (Bourne Again Shell) is basically a command language interpreter. It was written by Brian Fox for GNU OS and can be used in PLACE of Bourne Shell. It is similar to Bourne Shell but INCLUDES some additional features such as command-line editing that make it easier and more convenient to use. It is the default user shell on most Linux installations. It is basically an INTERPRETED and non-compiled PROCESS that can also run in the terminal window. It is also capable of reading commands from shell scripts.

LINUX generally consists of five basic elements or components as given below: 

• Kernel: It is considered a core or main part of Linux and is generally responsible for all major activities of OS such as process management, device management, etc.  
• System Library: These are special functions or PROGRAMS with the help of which application programs or system utilities can access features of the kernel without any requirement of code. It is simply used to implement the FUNCTIONALITY of the OS. 
• System Utility: These are utility programs that are responsible to perform specialized and individual-level TASKS. They are considered more liable and ALLOW users to manage the computer.  
• Hardware: It is physical hardware that includes items such as a mouse, keyboard, display, CPU, etc. 
• Shell: It is an environment in which we can run our commands, shell scripts, and programs. It is an interface between user and kernel that hides all complexities of functions of the kernel from the user. It is used to execute commands.